Alcohols, such as methyl alcohol, ethyl alcohol, and isopropyl alcohol (IPA), are used in large amounts as cleaning agents, solvents and synthetic raw materials for the chemical industry. Particularly, in the manufacturing step of a semiconductor device, a large amount of IPA is used in applications such as washing and drying. For example, an IPA evaporation drying method for performing water removal after performing pure water washing on a semiconductor device is effective as the process of performing water removal. On the other hand, a problem of the IPA evaporation drying method is that IPA, which has high volatility and of which high purity is required, is used, and as a result, the manufacturing cost of the semiconductor device increases. Therefore, in terms of cost reduction and an improvement in environmental load, it is desired to recover and reuse waste IPA which has been used in the semiconductor device manufacturing process. The IPA discharged from the manufacturing process of a semiconductor device contains impurities derived from the manufacturing process, the materials, and the equipment. In order to recover and reuse the IPA, it is necessary to remove these impurities to a high degree and purify the IPA to the same level as that in a case when it is purchased from the market for use in a semiconductor device manufacturing process. Main examples of the components of the impurities include water, ionic impurities, metals, and fine particles.
In commercial IPA, grades are set according to its applications, for example, use in a semiconductor device manufacturing process, and a standard value for each impurity is determined for each grade.
As a method of purifying an alcohol that is contaminated and contains impurities, a distillation method is known. However, when an attempt is made to purify an alcohol to a predetermined purity using only the distillation method, large-scale distillation equipment is required, and the equipment cost and the installation area increase, and enormous energy is required, and therefore, the energy cost also increases, which is not preferable in terms of economy.
For each impurity that may be contained in an alcohol, methods of removing the impurities from an alcohol are proposed as shown below.
For example, as a method of efficiently removing water in an alcohol, JP 11-276801A (Patent Literature 1) shows a method of setting water concentration in an alcohol to a certain level or less using a pervaporation (PV) method, and then removing water by adsorption using an adsorbent, such as zeolite. JP 6-69175A (Patent Literature 2) shows that water is separated from an alcohol using an anion exchange membrane as a separation membrane in a vapor permeation (VP) method, and the alcohol is further purified by distillation.
In the pervaporation method, a separation membrane having an affinity for a component (for example, water) that is the target of separation treatment is used, a mixed liquid containing the target component is flowed on the supply side of the separation membrane, and the pressure is reduced or an inert gas is flowed on the permeation side of the separation membrane to perform separation by the difference in permeation rate between each components in the separation membrane. The mixed liquid containing the target component is, for example, an alcohol which contains water as an impurity. Separation in a case where the fluid in contact with the membrane is in a gas phase is particularly referred to as the vapor permeation method. Separation in a case where the contact fluid is a liquid is referred to as the pervaporation method. In Patent Literature 1, as the separation membrane, a polyimide-based separation membrane or a cellulose-based separation membrane is used. In addition, as separation membranes for the dehydration of alcohols, zeolite membranes are also widely used. Zolite membranes are characterized in that: they have extremely strong water absorbency; regarding the adsorption of polar molecules, such as water molecules, they have high separation performance even when the partial pressure of the molecular species is extremely low; and the loss of the alcohol, the intended material, is small.
As methods of removing ionic impurities from an alcohol, methods using ion exchange resins are known as shown in JP 2009-57286A (Patent Literature 3), and Partha V. Buragohain, William N. Gill, and Steven M. Cramer; “Novel Resin-Based Ultrapurification System for Reprocessing IPA in the Semiconductor Industry,” Ind. Eng. Chem. Res., 1996, 35(9), pp. 3149-3154 (Non-patent Literature 1). Treatment by an ion exchange resin is characterized in that: the energy and the equipment cost are smaller and the treatment is simpler than those in a case where a distillation apparatus is used; and an alcohol having high purity can be obtained. In the method using an ion exchange resin, an alcohol-containing liquid is passed through an ion exchange resin layer. In addition, JP 2005-263729A (Patent Literature 4) proposes a method of removing cationic impurities, such as metal ions, and fine particles by using an ion exchange membrane instead of an ion exchange resin layer and combining a filter and the ion exchange membrane.
JP 9-57069A (Patent Literature 5) discloses that distillation is further performed on an alcohol from which water has been removed by the pervaporation method, to remove metals, and the alcohol is then passed through a microfiltration membrane to remove insoluble fine particles.
JP 2003-112060A (Patent Literature 6) discloses a regeneration system adapted to combine various methods as described above, purify IPA recovered from a semiconductor device manufacturing process, and supply it to the semiconductor device manufacturing process again. Patent Literature 6 also discloses a purification method in such a regeneration system. In the system described in Patent Literature 6, a plurality of water removal units are provided, and water removal is repeatedly performed so that the content of water in a waste chemical reaches the raw material level of the chemical.